Multicore fiber for communication

ABSTRACT

A multicore fiber for communication  10  which allows propagation of an optical signal includes: a clad  12 ; a core  11   a  which is arranged in a center of the clad  12 ; and seven to ten cores  11   b  which are arranged at equal intervals surrounding the core  11   a , and the cladding diameter is 230 μm, distances between centers of the mutually neighboring cores  11   a  and  11   b  are 30 μm or more, distances between the centers of the cores  11   b  and an outer peripheral surface of the clad  12  are 35 μm or more and a mode field diameter of light propagating in the cores  11   a  and  11   b  is 9 μm to 13 μm.

This application is a divisional of U.S. application Ser. No. 14/194,187filed on Feb. 28, 2014, which is based upon and claims the benefit ofpriority of the prior International Application No. PCT/JP2012/072510,filed on Sep. 4, 2012, and is based upon and claims the benefits ofpriority from Japanese Patent Application No. 2011-193403 filed on Sep.5, 2011, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a multicore fiber for communication inwhich more cores can be arranged under conditions that the multicorefiber for communication can be laid while reliability is secured.

BACKGROUND ART

Currently, an optical fiber used for an optical fiber communicationsystem which is generally spreading adopts a structure in which an outerperiphery of a core is surrounded by a clad, and an optical signalpropagates in this core to transmit information. Further, as the opticalcommunication system spreads in recent years, the amount of informationto be transmitted is remarkably increasing. As the amount of informationto be transmitted increases, the optical fiber communication system usesseveral tens to several hundreds of multiple optical fibers to performlong distance optical communication of large capacity.

To reduce the number of optical fibers in such an optical fibercommunication system, it is known as disclosed in, for example,following Non-Patent Document 1 that a plurality of signals istransmitted as light propagating in cores using a multicore fiber inwhich outer peripheries of a plurality of cores are surrounded by aclad.

CITATION LIST Non Patent Document

-   Non-Patent Document 1: Masanori KOSHIBA “Heterogeneous multi-core    fibers: proposal and design principle” IEICE Electronics Express,    Vol. 6, No. 2

SUMMARY OF INVENTION Objects to be Achieved by the Invention

In recent years, to increase the amount of information to betransmitted, a multicore fiber is demanded which has a greater number ofcores than a multicore fiber which is disclosed in Non-Patent Document 1and in which cores are provided in a 1-6 arrangement (an arrangement inwhich a core is arranged in the center, and six cores are arranged atequal intervals surrounding the core). As a multicore fiber which has agreater number of cores than the multicore fiber in which the cores areprovided in the 1-6 arrangement, Non-Patent Document 1 discloses amulticore fiber in which cores are provided in a 1-6-12 arrangement (anarrangement in which a core is arranged in the center, six cores arearranged at equal intervals surrounding this core, and twelve cores arefurther arranged at equal intervals surrounding these six cores).However, the multicore fiber in which the cores are provided in the1-6-12 arrangement has a concern that distances between the outerperipheral surface of the clad and the cores are too close and, in thiscase, microbend loss is likely to occur, and therefore reliability ofthe multicore fiber decreases.

Hence, distances between cores may be increased by increasing a claddingdiameter of a multicore fiber. However, when the cladding diameter isincreased too much, if the multicore fiber is bent and laid, themulticore fiber is easily fractured and reliability decreases in somecases.

It is therefore an object of the present invention to provide amulticore fiber for communication in which more cores can be arrangedunder conditions that the multicore fiber for communication can be laidwhile reliability is secured.

Means for Achieving the Objects

Conventionally, an allowable minimum bend radius of an optical fiber fortransmitting an optical signal is 30 mm or more from the view point ofbend loss characteristics. Meanwhile, in recent years, a fiber is beingdeveloped which causes little loss of light (bend loss) due to a bend,causes no bend loss even when the bend radius is 15 mm or less and isdurable against a bend. However, it is known that, if a long period oftime passes in a state where a portion having a small diameter is bent,the optical fiber is fractured due to a slight scratch in silica glass.Hence, the minimum bend radius which an optical fiber which is durableagainst a bend requires is determined prioritizing conditions based on aviewpoint of reliability over conditions based on a viewpoint of bendloss. Further, as an index for securing reliability of an optical fiberwhich is durable against a bend, an optical fiber in which a claddingdiameter is 125 μm requires a condition that the bend radius be lessthan 30 mm, and a fracture probability in twenty years is preferably1.0×10⁻⁷ or less in a state where the bend radius is 15 mm and thenumber of winding is 100. Meanwhile, a screening level of an opticalfiber assumes an elongation strain of 1%. Hence, for an optical fiber inwhich an cladding diameter is 125 μm or more, the bend radius which isconventionally employed is used and the above condition that the numberof winding is 100 which is the condition of the optical fiber in whichthe cladding diameter is 125 μm is used, and, as long as the fractureprobability in twenty years is 1.0×10⁻⁷ or less in a state where thebend radius is 30 mm and the number of winding is 100, it is possible tosecure sufficient reliability for a communication optical fiber.Further, the inventors of the present invention found that the claddingdiameter needs to be 230 μm or less to satisfy the condition that thefracture probability in twenty years is 1.0×10⁻⁷ or less in the statewhere the bend radius is 30 mm and the number of winding is 100. Hence,the inventors of the present invention made diligent study to arrangemore cores in a clad which has such an outer shape, and made the presentinvention.

That is, an aspect of the present invention includes: a clad; a corewhich is arranged in a center of the clad; and seven to twelve coreswhich are each arranged at equal distances from the center and at equalintervals surrounding the core, and an cladding diameter is 230 μm orless.

According to this multicore fiber for communication, the claddingdiameter is 230 μm or less, so that it is possible to secure reliabilityfor fracture caused when the multicore fiber for communication is laid.Compared to an optical fiber in which cores are provided in a 1-6-12arrangement, it is possible to increase distances between outermostperiphery side cores and an outer peripheral surface of the clad whenthe distances between cores are the same as those of this multicorefiber. Consequently, it is possible to suppress microbend loss, andsecure reliability.

Further, while, in a multicore fiber in which cores are provided in a1-6 arrangement, a core which is arranged in the center and two mutuallyneighboring cores which are arranged on an outer side form a regulartriangle, the present invention has seven or more the outer peripheryside cores, so that the core which is arranged in the center and twomutually neighboring cores are arranged on the outer side form anisosceles triangle. Hence, distances between centers of the center coreand the outer periphery side cores are greater than the distancesbetween the centers of the mutually neighboring outer periphery sidecores. When the distances between the cores increase, crosstalkdecreases, and crosstalk between the center core and the outer peripheryside cores is less than crosstalk between the outer periphery sidecores. Hence, when optical signals enter all cores, although a centercore having a greater number of the closest cores includes a greatertotal sum of crosstalk, it is possible to suppress a total sum ofcrosstalk of the center core according to the relationship.

Further, even though such a multicore fiber for communication cantransmit optical signals in this way without deteriorating the opticalsignals, eight or more cores are arranged therein, so that it ispossible to arrange more cores and transmit a greater amount ofinformation than a conventional common multicore fiber in which coresare provided in the 1-6 arrangement.

Furthermore, another aspect of the present invention includes: a clad;three cores which are each arranged at equal distances from a center ofthe clad and at equal intervals; and five to twelve cores which are eacharranged at equal distances from the center and at equal intervalssurrounding the core, and an cladding diameter is 230 μm or less.

Eight or more cores are also arranged according to such a corearrangement, so that it is possible to arrange more cores and transmit agreater amount of information than a conventional common multicore fiberin which cores are provided in the 1-6 arrangement. Further, compared tothe multicore fiber in which the cores are provided in the 1-6-12arrangement, it is possible to suppress microbend loss more and securereliability when the distances between the cores are the same as thoseof this multicore fiber.

Furthermore, still another aspect of the present invention is amulticore fiber for communication which allows propagation of an opticalsignal, and includes: a clad; a core which is arranged in a center ofthe clad; and six cores which are each arranged at equal distances fromthe center and at equal intervals surrounding the core; and one to sixcores which are arranged between a connection line which connects twomutually neighboring cores of the six cores and the core, and on theline which is vertical to the connection line and passes on the core,and an cladding diameter is 230 μm or less.

According to such a multicore fiber for communication, the cores arearranged in three layers, and one to six cores are arranged in thesecond layer. That is, the cores are arranged in a 1-6-6 arrangementfrom the 1-1-6 arrangement. Consequently, eight to thirteen cores arearranged as a whole, so that it is possible to arrange more cores andtransmit a greater amount of information than the conventional commonmulticore fiber in which cores are provided in a 1-6 arrangement.Further, compared to the multicore fiber in which the cores are providedin the 1-6-12 arrangement, it is possible to suppress microbend lossmore and secure reliability when the distances between the cores are thesame as those of this multicore fiber.

Furthermore, another aspect of the present invention includes: a clad; acore which is arranged in a center of the clad; and eight cores whichsurround the core and are arranged at equal intervals to forma squareshape as a whole, and an cladding diameter is 230 μm or less.

According to such a core arrangement, it is also possible to arrangemore cores and transmit a greater amount of information than aconventional common multicore fiber in which cores are provided in the1-6 arrangement. Further, compared to the multicore fiber in which thecores are provided in the 1-6-12 arrangement, it is possible to suppressmicrobend loss more and secure reliability when the distances betweenthe cores are the same as those of this multicore fiber.

Furthermore, still another aspect of the present invention includes: aclad; four cores which are each arranged at equal distances from acenter of the clad and at equal intervals; and eight cores which arearranged on an extended line which connects two mutually neighboringcores of the four cores, and which are arranged such that distances to acenter of a closest core of the four cores are equal to distancesbetween centers of the two mutually neighboring cores of the four cores,and an cladding diameter is 230 μm or less.

According to such a core arrangement, it is also possible to arrangemore cores and transmit a greater amount of information than aconventional common multicore fiber in which cores are provided in the1-6 arrangement. Further, compared to the multicore fiber in which thecores are provided in the 1-6-12 arrangement, it is possible to suppressmicrobend loss more and secure reliability when the distances betweenthe cores are the same as those of this multicore fiber.

Furthermore, in any one of the invention, a mode field diameter of lightpropagating in each of the cores is 9 μm to 13 μm, and a distancebetween centers of the cores which are mutually neighboring is 30 μm ormore, and a distance between the center of the core and an outerperipheral surface of the clad is preferably 35 μm or more.

When information is transmitted using the multicore fiber, the modefield diameter is about 9 μm to 13 μm, so that it is possible tosuppress loss of an optical signal. This is because the mode fielddiameter is 9 μm or more, so that it is possible to prevent loss oflight from increasing due to a non-linear optical effect because ofconcentration of light on cores, and the mode field diameter is 13 μm orless, so that it is possible to prevent loss of light from increasingdue to an increase of the amount of light leaking to an outside of thecores. Further, the mode field diameter is to 9 μm to 13 μm and thedistance between the centers of the mutually neighboring cores is 30 μmor more, so that it is possible to suppress crosstalk. Furthermore, themode field diameter is 9 μm to 13 μm and the distance between the centerof the core and the outer peripheral surface of the clad is 35 μm ormore, so that it is possible to prevent loss of an optical signal due toabsorption of light in a protective layer which covers the clad.

Still further, the invention preferably includes: an inner clad layerwhich has a lower refractive index than that of the core and whichsurrounds the core; and a low refractive index layer which has a loweraverage refractive index than those of the clad and the inner clad layerand which surrounds the inner clad layer. Each low refractive indexlayer surrounds each core across the inner clad layer, so that an effectof light confinement in each core is great and light hardly leaks fromthe cores. Consequently, it is possible to reduce crosstalk between thecores.

Further, in the invention, the low refractive index layer is made of amaterial having a lower refractive index than those of the clad and theinner clad layer. As to the refractive index of such a low refractiveindex layer, when each core element is viewed from the view point of therefractive index, the low refractive index layer has a trench shapewhich is referred to a “trench structure”. Such a structure is adopted,so that it is possible to suppress loss resulting from a bend of a fibermore, and a manufacturing method for mass production has already beenestablished, so that it is possible to easily manufacture multicorefibers for communication at low cost.

Alternatively, in the above invention, in the low refractive indexlayer, a plurality of low refractive index portions which has lowerrefractive indices than those of the clad and the inner clad layer isformed to surround the inner clad layer. In such a low refractive indexlayer, the low refractive index portions are not continuously formedannularly to surround the cores, so that it is possible to prevent eachcore from intensifying confinement of a higher mode, and prevent acutoff wavelength of each core from becoming longer.

Further, the low refractive index portion may be a hole. In this case,it is possible to make the refractive index of the low refractive indexlayer lower, and further reduce crosstalk while preventing each corefrom intensifying confinement of a higher mode.

Effect of Invention

As described above, the present invention provides a multicore fiber forcommunication in which more cores can be arranged under conditions thatthe multicore fiber for communication can be laid while reliability issecured.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating a structure of a vertical cross section ofa multicore fiber for communication in a longitudinal directionaccording to a first embodiment of the present invention.

FIG. 2 is a view illustrating a relationship between the bend radius andthe failure probability of an optical fiber.

FIG. 3 is a view illustrating a relationship between an claddingdiameter and distances between outer periphery side cores and an outerperipheral surface of the clad when one core is arranged in the centerand a plurality of cores is arranged around this core.

FIG. 4 is a view illustrating a structure of a vertical cross section ofa multicore fiber for communication in a longitudinal directionaccording to a second embodiment of the present invention.

FIG. 5 is a view illustrating a structure of a vertical cross section ofa multicore fiber for communication in a longitudinal directionaccording to a third embodiment of the present invention.

FIG. 6 is a view illustrating a relationship between an claddingdiameter and distances between outer periphery side cores and an outerperipheral surface of the clad when one core is arranged in the center,six cores are arranged around this core and a plurality of cores isarranged around these six cores at equal intervals.

FIG. 7 is a view illustrating a structure of a vertical cross section ofa multicore fiber for communication in a longitudinal directionaccording to a fourth embodiment of the present invention.

FIG. 8 is a view illustrating a structure of a vertical cross section ofa multicore fiber for communication in a longitudinal directionaccording to a fifth embodiment of the present invention.

FIG. 9 is a view illustrating a structure of a vertical cross section ofa multicore fiber for communication in a longitudinal directionaccording to a six embodiment of the present invention.

FIG. 10 is a view illustrating a structure of a vertical cross sectionof a multicore fiber for communication in a longitudinal directionaccording to a seventh embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Suitable embodiments of a multicore fiber for communication (referred toas a “multicore fiber” below) according to the present invention will beexplained in detail hereinafter referring to the drawings.

First Embodiment

FIG. 1 is a view illustrating a structure of a vertical cross section ofa multicore fiber in a longitudinal direction according to a firstembodiment of the present invention. As illustrated in FIG. 1, amulticore fiber 10 according to the present embodiment has a clad 12, acore 11 a which is arranged in the center in a cross section of the clad12, nine cores 11 b which are arranged at equal intervals surroundingthe core 11 a, an inner protective layer 13 which covers an outerperipheral surface of the clad 12 and an outer protective layer 14 whichcovers an outer peripheral surface of the inner protective layer 13.That is, in the multicore fiber 10 according to the present embodiment,a plurality of cores 11 a and 11 b is provided in a 1-9 arrangement.

This multicore fiber 10 is an optical fiber which allows propagation ofan optical signal, and a mode field diameter of light propagating in thecores 11 a and 11 b is preferably 9 μm to 13 μm. The mode field diameteris 9 μm or more, so that it is possible to prevent loss of light fromincreasing due to a non-linear optical effect because of concentrationof light on cores, and the mode field diameter is 13 μm or less, so thatit is possible to prevent loss of light from increasing due to anincrease of the amount of light leaking to an outside of the cores.

The clad 12 has a virtually circular shape as an outer shape in thecross section, and surrounds the outer peripheral surfaces of aplurality of cores 11 a and 11 b without gaps. A material for formingthe clad 12 is not limited in particular as long as the material isgenerally used for a clad, and the material is made of, for example,pure silica glass doped without a dopant.

The respective cores 11 a and 11 b are preferably arranged such thatdistances between centers of the mutually neighboring cores 11 a and 11b are 30 μm or more and, more preferably, 40 μm or more. The cores 11 aand 11 b are arranged in this way, so that it is possible to suppresscrosstalk between the mutually neighboring cores 11 a and 11 b.Particularly when the mode field diameter is 9 to 13 μm as describedabove, it is possible to prevent crosstalk by arranging the cores 11 aand 11 b in this way. Further, the cores 11 b arranged on the outerperiphery side are preferably arranged such that distances between thecenters of the cores 11 b and the outer peripheral surface of the clad12 are 35 μm or more and, more particularly, 40 μm or more. The cores 11b are arranged on the outer periphery side in this way, so that it ispossible to prevent loss of an optical signal caused when lightpropagating in the core 11 b on the outer periphery side is absorbed bythe inner protective layer 13.

Further, the respective cores 11 b arranged on the outer periphery sideare arranged at equal distances from the center of the clad 12 and atequal intervals. The cores 11 a and 11 b arranged in this way aresymmetrical with respect to a center axis of the clad 12. That is, whenthe multicore fiber 10 is rotated around the center axis of the clad 12at a predetermined angle, positions of the outer periphery side cores 11b after rotation come to positions of the other outer periphery sidecores 11 b before rotation. Further, the core 11 a arranged in thecenter does not move even when the multicore fiber 10 is rotated aboutthe center axis. The respective cores 11 a and 11 b are arranged atpositions symmetrical with respect to the center axis of the clad 12 inthis way, so that it is possible to make an optical property resultingfrom the arrangement of the cores 11 a and 11 b uniform.

Further, the distances between the centers of the center core 11 a andthe outer side cores 11 b are made longer than the distances between thecenters of the mutually neighboring outer side cores 11 b to draw anisosceles triangle with the core 11 a and the two mutually neighboringcores 11 b arranged in this way.

Furthermore, the diameters of the respective cores 11 a and 11 b are notlimited in particular and, for example, 8.7 μm to 12 μm. In addition,the diameters of the mutually neighboring cores 11 a and 11 b arepreferably slight different from each other. In this case, for example,the diameters of the cores 11 b arranged on the outer periphery side areabout 3% different from the diameter of the core 11 a arranged in thecenter, and the diameters of the mutually neighboring cores 11 barranged on the outer periphery side are about 0.5% to 5% different fromeach other. Thus, although, even when the diameters of the mutuallyneighboring cores 11 a and 11 b are physically slightly different, thecores 11 a and 11 b have only slightly different diameters for lightpropagating in the cores 11 a and 11 b and have the substantially sameoptical characteristics, the diameters of the mutually neighboring cores11 a and 11 b are physically slightly different, so that it is possibleto prevent crosstalk between the mutually neighboring cores 11 a and 11b.

Further, the refractive indices of the cores 11 a and 11 b are higherthan the refractive index of the clad 12, and a relative refractiveindex difference Δ with respect to the clad of the respective cores 11 aand 11 b is not limited in particular and is, for example, 0.21% to0.5%.

In addition, the refractive indices of mutually neighboring cores of therespective cores 11 a and 11 b are preferably different from each otherfrom the viewpoint of preventing crosstalk between the respective cores11 a and 11 b. In this case, the differences between the refractiveindices of the mutually neighboring cores 11 a and 11 b are preferably1% to 5% of the refractive indices from the view point of preventingcrosstalk and making optical characteristics of the cores equal.

A material of these cores 11 a and 11 b is not limited in particular aslong as the material has the refractive index which is higher than theclad 12 and is the above relative refractive index difference withrespect to the clad, and is, for example, silica glass doped with adopant such as germanium which increases the refractive index.

Further, a material of the inner protective layer 13 and the outerprotective layer 14 includes ultraviolet curable resin of differenttypes from each other.

In addition, this multicore fiber 10 can be manufactured by a stack anddraw method. That is, a plurality of rod-shaped core glass members whichform the cores 11 a and 11 b, a tubular clad glass member which formspart of the clad 12 and a rod-shaped clad glass member which forms partof the clad 12 are prepared first. Further, the respective core glassmembers are arranged in penetration holes of the tubular clad glassmember, and the rod-shaped clad glass member is arranged between thetubular clad glass member and the core glass members to fill gaps.Furthermore, these members are collapsed in a state where the core glassmembers are arranged to make a fiber base material in which anarrangement in the cross section has a similar figure without the innerprotective layer 13 and the outer protective layer 14 in the multicorefiber 10 illustrated in FIG. 1. Still further, the created fiber basematerial is heated, melted and spun to make a multicore fiber, and thismulticore fiber is covered by the inner protective layer 13 and theouter protective layer 14 to make the multicore fiber 10. Alternatively,the core glass members and the rod-shaped clad glass member may be spunwhile being collapsed in a state where the core glass members and therod-shaped clad glass member are arranged in penetration holes of thetubular clad glass member.

Alternatively, in an intermediate base material which has in the centerthe core glass member which forms the core 11 a, penetration holes aremade around the core glass member which is the center of the clad glassmember, glass rods which have the core glass members are inserted inthese penetration holes, and the these glass rods and the clad glassmembers are collapsed filling the gaps therebetween to make the samefiber base material as the above. Further, the members are spun in thesame way as the above and covered by the inner protective layer 13 andthe outer protective layer 14 to make the multicore fiber 10.Alternatively, the members may be spun while being collapsed in a statewhere the above glass rods are inserted in the above penetration holes.

Next, the cladding diameter will be described. Conventionally, anallowable minimum bend radius of an optical fiber for transmitting anoptical signal is 30 mm or more from the view point of bend losscharacteristics. Meanwhile, in recent years, a fiber is being developedwhich causes little loss of light (bend loss) due to a bend, causes nobend loss even when the bend radius is 15 mm or less and is durableagainst a bend. However, it is known that, if a long period of timepasses in a state where a portion having a small diameter is bent, theoptical fiber is fractured due to a slight scratch in silica glass.Hence, the minimum bend radius which an optical fiber requires isdetermined prioritizing conditions based on a view point of reliabilityover conditions based on a viewpoint of bend loss. Further, as an indexfor securing reliability, an optical fiber in which an cladding diameteris 125 μm requires a condition that the bend radius be less than 30 mm,and a fracture probability in twenty years is preferably 1.0×10⁻⁷ orless in a state where the bend radius is 15 mm and the number of windingis 100. Meanwhile, a screening level of an optical fiber assumes anelongation strain of 1%. Hence, for an optical fiber in which ancladding diameter is 125 μm or more, the bend radius which isconventionally employed is used and the above condition that the numberof winding is 100 which is the condition of the optical fiber in whichthe cladding diameter is 125 μm is used, and, as long as the fractureprobability in twenty years is 1.0×10⁻⁷ or less in a state where thebend radius is 30 mm and the number of winding is 100, it is possible tosecure sufficient reliability for a communication optical fiber.

FIG. 2 is a view illustrating a relationship between the bend radius andthe fracture probability of an optical fiber. FIG. 2 illustrates curvesillustrating relationships between bend radii and fracture probabilitiesin twenty years when optical fibers in which cladding diameters are 125μm, 150 μm, 200 μm, 230 μm, 250 μm and 300 μm are wound a hundred times.

As illustrated in FIG. 2, when the diameter is 230 μm or less, thefracture probability in the twenty years in case that the optical fiberis wound a hundred times is 1.0×10⁻⁷ or less. Hence, to lay themulticore fiber while reliability is secured, the cladding diameter ofthe multicore fiber 10 is 230 μm or less. Further, when the distancesbetween the centers of the mutually neighboring ores 11 a and 11 b are30 μm or more and the distances between the centers of the cores 11 barranged on the outer periphery side and the outer peripheral surface ofthe clad 12 are 35 μm or more, the cladding diameter is 158 μm to 230 μmin the multicore fiber 10 in which the cores are provided in a 1-9arrangement as in the present embodiment. Furthermore, when thedistances between the centers of the mutually neighboring cores 11 a and11 b are 40 μm or more and the distances between the centers of thecores 11 b arranged on the outer periphery side and the outer peripheralsurface of the clad 12 are 40 μm or more, the cladding diameter is 197μm to 230 μm in the multicore fiber 10 in which the cores are providedin a 1-9 arrangement as in the present embodiment.

As described above, according to this multicore fiber 10, the claddingdiameter is 230 μm or less, so that it is possible to secure reliabilityfor fracture caused when the multicore fiber 10 is laid. Further,compared to an optical fiber in which cores are provided in a 1-6-12arrangement, it is possible to increase distances between the centers ofthe outermost periphery side cores 11 b and an outer peripheral surfaceof the clad 12 when the distances between cores are the same as those ofthis multicore fiber. Consequently, it is possible to suppress microbendloss, and secure reliability.

Further, the multicore fiber 10 has reliability as described above andhas 10 cores arranged therein, and, consequently, can transmit a greateramount of information than a conventional common multicore fiber inwhich cores are provided in a 1-6 arrangement.

Furthermore, while, in a multicore fiber in which cores are provided inthe 1-6 arrangement, a core which is arranged in the center and twomutually neighboring cores which are arranged on an outer side form aregular triangle, the multicore fiber 10 according to the presentinvention has seven or more outer periphery side cores 11 b, so that thecore 11 a arranged in the center and two mutually neighboring coresarranged on the outer side form an isosceles triangle. Hence, distancesbetween the centers of the center core 11 a and the outer periphery sidecores 11 b are greater than the distances between the centers of themutually neighboring outer periphery side cores 11 b. When the distancesbetween the cores increase, crosstalk decreases, and crosstalk betweenthe center core 11 a and the outer periphery side cores 11 b is lessthan crosstalk between the outer periphery side cores 11 b. Hence, whenoptical signals enter all cores 11 a and 11 b, although the center core11 a having a greater number of the closest cores includes a greatertotal sum of crosstalk, it is possible to suppress a total sum ofcrosstalk of the center core 11 a according to the relationship. It ispossible to balance the crosstalk between the cores 11 a and 11 b of themulticore fiber 10 as a whole.

Further, when the distances between the centers of the mutuallyneighboring cores 11 a and 11 b are 30 μm or more, it is possible toreduce crosstalk and, when the distances are 40 μm or more, it ispossible to further reduce crosstalk, so that it is possible to securehigher reliability. Furthermore, when the distances between the centersof the cores 11 b and the outer peripheral surface of the clad 12 are 35μm or more, loss of a optical signal caused by absorption of light ininner protective layer 13 can be prevented and, consequently, it ispossible to secure reliability for communication and, when the distancesare 40 μm or more, it is possible to secure higher reliability.

In addition, although an example has been described with the presentembodiment where a core is arranged in the center and nine cores arearranged on an outer periphery side, the number of outer periphery sidecores is not limited to this, and the number of cores can be adequatelychanged as long as the number of cores is greater than that of theconventional common multicore fiber in which the cores are provided inthe 1-6 arrangement. FIG. 3 illustrates a relationship between thecladding diameter, and distances between the outer periphery side coresand the outer peripheral surface of the clad when the distances betweenthe mutually neighboring cores are 40 μm in a multicore fiber in which acore is arranged in the center and a plurality of cores is arrangedaround this core at equal intervals.

As illustrated in FIG. 3, the leftmost line illustrates a relationshipbetween the cladding diameter, and the distances between the centers ofthe outer periphery side cores 11 b and the outer peripheral surface ofthe clad 12 when the cores 11 a and 11 b are provided in the 1-6arrangement. Further, lines right to this line illustrate relationshipsbetween the cladding diameter, and the distances between the centers ofthe outer periphery side cores 11 b and the outer peripheral surface ofthe clad 12 when the cores 11 a and 11 b are provided in a 1-7arrangement, a 1-8 arrangement, a 1-9 arrangement, a 1-10 arrangement, a1-11 arrangement and a 1-12 arrangement in this order.

As described above, the cladding diameter needs to be 230 μm or lessfrom the viewpoint of securing reliability for fracture when themulticore fiber is laid, and the distances between the centers of thecores 11 b arranged on the outer periphery side and the outer peripherysurface of the clad 12 are preferably 35 μm or more and, morepreferably, 40 μm or more from the view point of suppressing loss of aoptical signal caused by absorption of light in the inner protectivelayer 13. In the multicore fiber in which the number of cores is greaterthan that of the conventional common multicore fiber in which the coresare provided in the 1-6 arrangement, when the core 11 a is arranged inthe center of the clad 12 and a plurality of cores 11 is arranged on theouter periphery side surrounding this core 11 a, to arrange a greaternumber of cores while reliability is secured for fracture when themulticore fiber is laid, the number of the outer periphery side cores 11b is seven to twelve when the distances between the centers of the cores11 b and the outer periphery surface of the clad 12 are 35 μm or lessand the number of the outer periphery side cores 11 b is seven to elevenwhen the distances between the centers of the cores 11 b and the outerperiphery surface of the clad 12 are 40 μm or less. In addition, whenthe cores 11 a and 11 b are provided in the 1-9 arrangement, the lowerlimit of the cladding diameter can also be calculated from FIG. 3.

Second Embodiment

Next, referring to FIG. 4, a second embodiment of the present inventionwill be described in detail. In addition, components that are identicalor similar to those in the first embodiment will be denoted by the samereference numerals as those used in the first embodiment unlessparticularly described, and will not be described.

FIG. 4 is a view illustrating a structure of a vertical cross section ofa multicore fiber in a longitudinal direction according to the secondembodiment of the present invention. As illustrated in FIG. 4, amulticore fiber 20 according to the present embodiment differs from amulticore fiber 10 according to the first embodiment in an arrangementof cores 11 a and 11 b. More specifically, three cores 11 a are arrangedat equal intervals surrounding a center C in a cross section of a clad12, and nine cores 11 b are arranged around the center C of the clad 12surrounding these three cores 11 a. That is, in the multicore fiber 20according to the present embodiment, a plurality of cores 11 a and 11 bis provided in a 3-9 arrangement.

The multicore fiber 20 according to the present embodiment is an opticalfiber which allows propagation of an optical signal similar to themulticore fiber 10 according to the first embodiment, and the mode fielddiameter of light propagating in the respective cores 11 a and 11 b ispreferably 9 μm to 13 μm for the same reason as for the multicore fiber10 according to the first embodiment.

Further, for the same reason as for the multicore fiber 10 according tothe first embodiment, distances between centers of the respective cores11 a and 11 b are preferably 30 μm or more and, more preferably, 40 μmor more. Furthermore, for the same reason as for the multicore fiber 10according to the first embodiment, distances between the centers of theouter periphery side cores 11 b and the outer periphery side of the clad12 are also preferably 35 μm or more and, more preferably, 40 μm ormore.

In the present embodiment, when the distances between the centers of themutually neighboring cores 11 a and 11 b are 30 μm or more and thedistances between the centers of the cores 11 b arranged on the outerperiphery side and the outer peripheral surface of the clad 12 are 35 μmor more as described above, the cladding diameter is 165 μm to 230 μm,and, when the distances between the centers of the mutually neighboringcores 11 a and 11 b are 40 μm or more and the distances between thecenters of the cores 11 b arranged on the outer periphery side and theouter peripheral surface of the clad 12 are 40 μm or more, the claddingdiameter is 207 μm to 230 μm. The reason why the upper limit of thecladding diameter is 230 μm is the same as the reason why the upperlimit of the cladding diameter of the multicore fiber 10 according tothe first embodiment is 230 μm.

Twelve cores are arranged according to the arrangement of the cores 11 aand 11 b of the multicore fiber 20 according to the present embodiment,so that it is possible to arrange more cores and transmit a greateramount of information than a conventional common multicore fiber inwhich cores are provided in a 1-6 arrangement.

In addition, when three cores are arranged surrounding the center C ofthe clad 12 as in the multicore fiber 20 according to the presentembodiment, if the number of outer periphery side cores 11 b surroundingthese three cores 11 a is five or more, it is possible to arrange morecores than the conventional common multicore fiber in which the coresare provided in the 1-6 arrangement. Further, the cladding diameter is230 μm or less, and therefore the number of the outer periphery sidecores 11 b is twelve or less. Hence, when the three cores are arrangedsurrounding the center C of the clad 12, the number of outer peripheryside cores is five to twelve. That is, when the three cores are arrangedin the center as in the present embodiment, the cores 11 a and 11 b areprovided in a 3-5 arrangement to a 3-12 arrangement to arrange morecores while reliability is secured for fracture when the multicore fiberis laid.

Third Embodiment

Next, referring to FIG. 5, a third embodiment of the present inventionwill be described in detail. In addition, components that are identicalor similar to those in the first embodiment will be denoted by the samereference numerals as those used in the first embodiment unlessparticularly described, and will not be described.

FIG. 5 is a view illustrating a structure of a vertical cross section ofa multicore fiber in a longitudinal direction according to a thirdembodiment of the present invention. As illustrated in FIG. 5, amulticore fiber 30 according to the present embodiment differs from amulticore fiber 10 according to the first embodiment in a core 11 aarranged in the center, a plurality of cores 11 b and, in addition, aplurality of cores 11 c arranged on an outermost periphery side. Morespecifically, the core 11 a is arranged in the center in the crosssection of a clad 12, the outermost periphery side six cores 11 c arearranged at equal intervals surrounding this core 11 a, and, betweenconnection lines Lc (indicated by broken lines in FIG. 5) which connectthe two mutually neighboring cores 11 c of the six cores 11 c and thecore 11 a arranged in the center of the clad 12, the cores 11 b arearranged on lines Lv (indicated by dashed lines in FIG. 5) which arevertical to the connection lines Lc and pass on the core 11 a. Hence,the six cores 11 b are arranged. That is, in the multicore fiber 30according to the present embodiment, a plurality of cores 11 a, 11 b and11 c is provided in a 1-6-6 arrangement. In addition, in the presentembodiment, the respective cores 11 a, 11 b and 11 c are arranged toform a triangular grid.

The multicore fiber 30 according to the present embodiment is an opticalfiber which allows propagation of an optical signal similar to themulticore fiber 10 according to the first embodiment, and the mode fielddiameter of light propagating in the respective cores 11 a, 11 b and 11c is preferably 9 μm to 13 μm for the same reason as for the multicorefiber 10 according to the first embodiment.

Further, for the same reason as for the multicore fiber 10 according tothe first embodiment, distances between centers of the respective cores11 a, 11 b and 11 c are preferably 30 μm or more and, more preferably,40 μm or more. Furthermore, for the same reason as for the multicorefiber 10 according to the first embodiment, distances between thecenters of the outer periphery side cores 11 c and the outer peripheryside of the clad 12 are also preferably 35 μm or more and, morepreferably, 40 μm or more.

FIG. 6 is a view illustrating a relationship between an claddingdiameter and distances between outer periphery side cores and an outerperipheral surface of the clad when one core is arranged in the center,six cores are arranged around this core and a plurality of cores isarranged around these six cores at equal intervals, and when thedistances between the centers of the mutually neighboring cores are 40μm. In FIG. 6, a left side line indicates a relationship between thecladding diameter and the distances between the outer periphery sidecores and the outer peripheral surface of the clad in case of a 1-6-6arrangement similar to the present embodiment, and a right side lineindicates a relationship between the cladding diameter and the distancesbetween the outer periphery side cores and the outer peripheral surfaceof the clad in case of a 1-6-12 arrangement. As illustrated in FIG. 6,as long as the cores are provided in a 1-6-6 arrangement as in thepresent embodiment, a line is positioned in an area in which thecladding diameter is 230 μm or less and the distances between thecenters of the cores 11 c arranged on the outer periphery side and theouter peripheral surface of the clad 12 are 40 μm or more. Meanwhile,according to the 1-6-12 arrangement, the line is not positioned in thisarea, and such an arrangement cannot be adopted.

In the present embodiment, when the distances between the centers of themutually neighboring cores 11 a, 11 b and 11 c are 30 μm or more and thedistances between the centers of the cores 11 b arranged on the outerperiphery side and the outer peripheral surface of the clad 12 are 35 μmor more as described above, the cladding diameter is 174 μm to 230 μm,and, when the distances between the centers of the mutually neighboringcores 11 a and 11 b are 40 μm or more and the distances between thecenters of the cores 11 b arranged on the outer periphery side and theouter peripheral surface of the clad 12 are 40 μm or more, the claddingdiameter is 219 μm to 230 μm as illustrated in FIG. 6. In addition, thereason why the upper limit of the cladding diameter is 230 μm is thesame as the reason why the upper limit of the cladding diameter of themulticore fiber 10 according to the first embodiment is 230 μm.

Thirteen cores are arranged according to the arrangement of the cores 11a, 11 b and 11 c of the multicore fiber 30 according to the presentembodiment, so that it is possible to arrange more cores and transmit agreater amount of information than a conventional common multicore fiberin which cores are provided in the 1-6 arrangement.

Further, although an example has been described with the presentembodiment where the number of cores 11 b is six, the number of cores 11b can be one to six. Hence, the core 11 a is arranged in the center ofthe clad 12, the six cores 11 c are arranged at equal intervalssurrounding this core 11 a and, between the connection lines Lcconnecting the two mutually neighboring cores 11 c of these six cores 11c and the core 11 a arranged in the center of the clad 12, at least onecore 11 b may be arranged on the line Lv which is vertical to theconnection line Lc and passes on the core 11 a arranged in the center ofthe clad 12. That is, when the core is arranged in the center as in thepresent embodiment and cores are arranged in three layers as a whole,the cores 11 a, 11 b and 11 c are provided in a 1-1-6 arrangement to a1-6-6 arrangement to arrange more cores while reliability is secured forfracture when the multicore fiber is laid.

Fourth Embodiment

Next, referring to FIG. 7, a fourth embodiment of the present inventionwill be described in detail. In addition, components that are identicalor similar to those in the first embodiment will be denoted by the samereference numerals as those used in the first embodiment unlessparticularly described, and will not be described.

FIG. 7 is a view illustrating a state of the multicore fiber accordingto the fourth embodiment of the present invention. As illustrated inFIG. 7, a multicore fiber 40 according to the present embodiment differsfrom a multicore fiber 10 according to the first embodiment in that aplurality of cores 11 a and 11 b in the multicore fiber 10 according tothe first embodiment is surrounded by inner clad layers 15 a and 15 bwithout gaps and, further, the inner clad layers 15 a and 15 b aresurrounded by the respective low refractive index layers 16 a and 16 b.In the present embodiment, the cores 11 a and 11 b, the inner cladlayers 15 a and 15 b and the low refractive index layers 16 a and 16 bform core elements 41 a and 41 b.

The outer diameters of the inner clad layers 15 a and 15 b are mutuallyequal, and the outer diameters of the respective low refractive indexlayers 16 a and 16 b are mutually equal. Hence, the thicknesses of therespective inner clad layers 15 a and 15 b are mutually equal, and thethicknesses of respective the low refractive index layers 16 a and 16 bare mutually equal.

Meanwhile, refractive indices n₅ of the respective inner clad layers 15a and 15 b and the refractive index n₂ of the clad 12 are lower than therefractive indices n₁ of the respective cores 11 a and 11 b, and therefractive indices n₆ of the low refractive index layers 16 a and 16 bare lower than the refractive indices n₅ of the inner clad layers 15 aand 15 b and the refractive index n₂ of the clad 12. In other words, therespective refractive indices n₁, n₂, n₅ and n₆ all satisfyn ₁ >n ₅ >n ₆,n ₁ >n ₂ andn ₆ <n ₂.

Further, in the present embodiment, the refractive indices n₅ of theinner clad layers 15 a and 15 b and the refractive index n₂ of the clad12 are mutually equal. That is, n₅=n₂ is true.

From the view point of the refractive indices of the respective coreelements 41 a and 41 b, the low refractive index layers 16 a and 16 bhave low shapes like trenches in the respective core elements 41 a and41 b, and the core elements 41 a and 41 b adopt the trench structures.In addition, in the present embodiment, the refractive indices of therespective low refractive index layers 16 a and 16 b are uniform in therespective low refractive index layers 16 a and 16 b, and the refractiveindices of the respective low refractive index layers 16 a and 16 b andthe average refractive index means the same.

In addition, the inner clad layers 15 a and 15 b have the refractiveindices equal to the clad 12 as described above, and therefore is formedusing, for example, the same material as that of the clad 12. Further,the low refractive index layers 16 a and 16 b are made of silica glassdoped with a dopant which decreases the refractive indices. Such adopant is, for example, fluorine.

In the respective core elements 41 a and 41 b of this multicore fiber40, the refractive indices n₆ of the low refractive index layers 16 aand 16 b are lower than the refractive indices n₅ of the inner cladlayers 15 a and 15 b and the refractive index n₂ of the clad 12, so thatan effect of light confinement in the cores 11 a and 11 b increases andlight hardly leaks from the cores 11 a and 11 b. Consequently, it ispossible to prevent light propagating in the cores 11 a and 11 b fromleaking from the core elements 41 a and 41 b. Further, the lowrefractive index layers 16 a and 16 b and the clad 12 having the lowrefractive indices serve as barriers, so that it is possible to preventcrosstalk between the cores 11 a and 11 b in the mutually neighboringcore elements.

Hereinafter, an example of the property according to the presentembodiment will be described. In this multicore fiber 40, the claddingdiameter is 204.4 μm, the distances between the centers of the centercore 11 a and outer periphery side cores 11 b are 59.2 μm, the distancesbetween the outer periphery side cores 11 b are 40.5 μm, the distancesbetween the centers of the outer periphery side cores 11 b and the outerperipheral surface of the clad 12 are 43 μm, the thicknesses of theinner clad layers 15 a and 15 b are 6.0 μm respectively, the thicknessesof the low refractive index layers 16 a and 16 b are 4.3 μm, therelative refractive index difference of the center core is 0.23% and therelative refractive index differences of the low refractive index layersare −0.65%.

In this multicore fiber 40, when an optical signal having a wavelengthin a 1.55 μm band propagates in the cores 11 a and 11 b, an effectivecross-sectional area Aeff of the center core 11 a is 116.04 μm², acutoff wavelength is 1.25 μm, the effective cross-sectional areas Aeffof the outer periphery side cores 11 b are 118.2 μm² to 125.2 μm², andcutoff wavelengths are 1.28 μm to 1.39 μm. Further, crosstalk betweenthe outer periphery side cores 11 b when a measurement length is 3.96 kmis −38.6 dB to −41.6 dB, and crosstalk between the center core 11 a andthe outer periphery side cores 11 b is −71.7 to −75.2 dB.

Fifth Embodiment

Next, referring to FIG. 8, a fifth embodiment of the present inventionwill be described in detail. In addition, components that are identicalor similar to those in the fourth embodiment will be denoted by the samereference numerals as those used in the fourth embodiment unlessparticularly described, and will not be described.

FIG. 8 is a view illustrating a structure of a vertical cross section ofa multicore fiber in a longitudinal direction according to the fifthembodiment of the present invention. As illustrated in FIG. 8, amulticore fiber 50 according to the present embodiment differs from amulticore fiber 40 according to the fourth embodiment in that, insteadof respective core elements 41 a and 41 b according to the fourthembodiment, core elements 51 a and 51 b arranged at the same spots asthose of the respective core elements 41 a and 41 b are arranged. Morespecifically, in the respective core elements 51 a and 51 b, lowrefractive index layers 17 a and 17 b are arranged instead of lowrefractive index layers 16 a and 16 b according to the fourthembodiment.

In the respective low refractive index layers 17 a and 17 b, a pluralityof low refractive index portions 18 which have lower refractive indicesthan those of the clad 12 and the inner clad layers 15 a and 15 b areformed surrounding the inner clad layers 15 a and 15 b. In the presentembodiment, a plurality of circular holes is formed in the lowrefractive index layers 17 a and 17 b, and these holes are the lowrefractive index portions 18. Hence, the shape in the cross section ofthe low refractive index portion 18 is circular. In the presentembodiment, the cores 11 a and 11 b, the inner clad layers 15 a and 15 band the low refractive index layers 17 a and 17 b form core elements 51a and 51 b.

Further, in the present embodiment, an area other than the lowrefractive index portions 18 in the respective low refractive indexlayer 17 a and 17 b is made of the same material as those of the clad 12and the inner clad layers 15 a and 15 b. Furthermore, the low refractiveindex portions 18 are holes, and the refractive indices of the lowrefractive index portions 18 are 1 and are lower than the refractiveindices of the inner clad layers 15 a and 15 b and the clad 12 and theaverage refractive index of the low refractive index layers 17 a and 17b is lower than those of the inner clad layers 15 a and 15 b and theclad 12.

In the multicore fiber 50 according to the present embodiment, the lowrefractive index portions 18 which have low refractive indices are notcontinuously annularly formed surrounding the respective cores 11 a and11 b, so that it is possible to allow a higher mode to adequately escapefrom the respective cores 11 a and 11 b. Consequently, it is possible toprevent the cutoff wavelengths of the cores 11 a and 11 b from becominglong.

Further, the low refractive index portions 18 are holes, so that it ispossible to make the refractive index of the low refractive index layers17 a and 17 b lower, and further reduce crosstalk between the respectivecores 11 a and 11 b while preventing the respective cores 11 a and 11 bfrom intensifying confinement of a higher mode.

In addition, although, in the present embodiment, the low refractiveindex portions 18 are formed as holes and, consequently, it is possibleto make the refractive indices of the low refractive index portions 18smaller, the low refractive index portions 18 are not limited to holes,and the material is not limited in particular as long as the materialhas a lower refractive index than those of the inner clad layers 15 aand 15 b and the clad 12. For example, the low refractive index portions18 may be formed using silica doped with a dopant such as fluorine whichdecreases the refractive index. Even in this case, it is possible toreduce the number of silica doped with fluorine which is costly in themulticore fiber 50 according to the present embodiment, and manufacturethe multicore fibers 50 at low cost.

Sixth Embodiment

Next, referring to FIG. 9, a sixth embodiment of the invention will bedescribed in detail. In addition, components that are identical orsimilar to those in the first embodiment will be denoted by the samereference numerals as those used in the first embodiment unlessparticularly described, and will not be described.

FIG. 9 is a view illustrating a structure of a vertical cross section ofa multicore fiber in a longitudinal direction according to the sixembodiment of the present invention. As illustrated in FIG. 6, amulticore fiber 60 according to the present embodiment differs from amulticore fiber 10 according to the first embodiment in the number andan arrangement of cores 11 b. More specifically, a core 11 a arranged ina center of a clad 12 is surrounded by eight cores 11 b, and these eightcores 11 b are arranged at equal intervals to form a square as a whole.

The multicore fiber 60 according to the present embodiment is an opticalfiber which allows propagation of an optical signal similar to themulticore fiber 10 according to the first embodiment, and the mode fielddiameter of light propagating in the cores 11 a and 11 b is preferably 9μm to 13 μm for the same reason as for the multicore fiber 10 accordingto the first embodiment.

Further, for the same reason as for the multicore fiber 10 according tothe first embodiment, distances between centers of the cores 11 a and 11b are preferably 30 μm or more and, more preferably, 40 μm or more.Furthermore, for the same reason as for the multicore fiber 10 accordingto the first embodiment, distances between the centers of the outerperiphery side cores 11 b and the outer periphery side of the clad 12are also preferably 35 μm or more and, more preferably, 40 μm or more.

In the present embodiment, when the distances between the centers of themutually neighboring cores 11 a and 11 b are 30 μm or more and thedistances between the centers of the cores 11 b arranged on the outerperiphery side and the outer peripheral surface of the clad 12 are 35 μmor more as described above, the cladding diameter is 155 μm to 230 μm,and, when the distances between the centers of the mutually neighboringcores 11 a and 11 b are 40 μm or more and the distances between thecenters of the cores 11 b arranged on the outer periphery side and theouter peripheral surface of the clad 12 are 40 μm or more, the claddingdiameter is 194 μm to 230 μm. The reason why the upper limit of thecladding diameter is 230 μm is the same as the reason why the upperlimit of the cladding diameter of the multicore fiber 10 according tothe first embodiment is 230 μm.

Ten cores are arranged according to the arrangement of the cores 11 aand 11 b of the multicore fiber 60 according to the present embodiment,so that it is possible to arrange more cores and transmit a greateramount of information than a conventional common multicore fiber inwhich cores are provided in the 1-6 arrangement.

Seventh Embodiment

Next, referring to FIG. 10, a seventh embodiment of the presentinvention will be described in detail. In addition, components that areidentical or similar to those in the first embodiment will be denoted bythe same reference numerals as those used in the first embodiment unlessparticularly described, and will not be described.

FIG. 10 is a view illustrating a structure of a vertical cross sectionof a multicore fiber in a longitudinal direction according to theseventh embodiment of the present invention. As illustrated in FIG. 10,a multicore fiber 70 according to the present embodiment differs from amulticore fiber 10 according to the first embodiment in the number andan arrangement of cores 11 a and 11 b. More specifically, the number ofcores 11 a is four, and the cores 11 a are arranged at equal distancesfrom a center C of a clad 12 and at equal intervals. Further, the numberof cores 11 b which surround these cores 11 a is eight, and the cores 11b are arranged one by one respectively, on extended lines Le (indicatedby broken lines in FIG. 10) connecting two mutually neighboring cores 11a of the four cores 11 a and are arranged such that a distance to thecenter of the closest core 11 of the four cores 11 a is equal to thedistances between the centers of the two mutually neighboring cores 11 aof the four cores 11 a. That is, when the distance between the centersof the two mutually neighboring cores 11 a is d, the distance betweenthe centers of the specific core 11 b and the core 11 a closest to thisspecific core 11 b is also d.

The multicore fiber 70 according to the present embodiment is an opticalfiber which allows propagation of an optical signal similar to themulticore fiber 10 according to the first embodiment, and the mode fielddiameter of light propagating in the cores 11 a and 11 b is preferably 9μm to 13 μm for the same reason as for the multicore fiber 10 accordingto the first embodiment.

Further, for the same reason as for the multicore fiber 10 according tothe first embodiment, distances between centers of the respective cores11 a and 11 b are preferably 30 μm or more and, more preferably, 40 μmor more. Furthermore, for the same reason as for the multicore fiber 10according to the first embodiment, distances between the centers of theouter periphery side cores 11 b and the outer periphery side of the clad12 are also preferably 35 μm or more and, more preferably, 40 μm ormore.

In the present embodiment, when the distances between the centers of themutually neighboring cores 11 a and 11 b are 30 μm or more and thedistances between the centers of the cores 11 b arranged on the outerperiphery side and the outer peripheral surface of the clad 12 are 35 μmor more as described above, the cladding diameter is 165 μm to 230 μm,and, when the distances between the centers of the mutually neighboringcores 11 a and 11 b are 40 μm or more and the distances between thecenters of the cores 11 b arranged on the outer periphery side and theouter peripheral surface of the clad 12 are 40 μm or more, the claddingdiameter is 207 μm to 230 μm. The reason why the upper limit of thecladding diameter is 230 μm is the same as the reason why the upperlimit of the cladding diameter of the multicore fiber 10 according tothe first embodiment is 230 μm.

Ten cores are arranged according to the arrangement of the cores 11 aand 11 b of the multicore fiber 70 according to the present embodiment,so that it is possible to arrange more cores and transmit a greateramount of information than a conventional common multicore fiber inwhich cores are provided in the 1-6 arrangement.

Although the present invention has been described above by reference tothe first to seventh embodiments as examples, the present invention isnot limited thereto.

For example, in the first embodiment to the third embodiment, the sixthembodiment and the seventh embodiment, these cores 11 a and 11 b may besurrounded by inner clad layers and low refractive index layers as inthe fourth embodiment and the fifth embodiment.

INDUSTRIAL APPLICABILITY

As describe above, the present invention provides a multicore fiber forcommunication in which more cores can be arranged under conditions thatthe multicore fiber for communication can be laid while reliability issecured, and is useful for communication optical fibers for shortdistance transmission disposed in households and communication opticalfibers for long distance transmission such as undersea cables.

REFERENCE SIGNS LIST

-   10, 20, 30, 40, 50, 60, 70 . . . Multicore fiber-   11 a, 11 b, 11 c . . . Core-   12 . . . Clad-   13 . . . Inner protective layer-   14 . . . Outer protective layer-   15 a, 15 b . . . Inner clad layer-   16 a, 16 b, 17 a, 17 b . . . Low refractive index layer-   18 . . . Low refractive index portion-   41 a, 41 b, 51 a, 51 b . . . Core element

The invention claimed is:
 1. A multicore fiber for communicationcomprising: a clad; three first cores which are each arranged at equaldistances from a center of the clad and at equal intervals, the threefirst cores each being surrounded by the clad; and five to twelve secondcores which are each arranged at equal distances from the center and atequal intervals surrounding the three first cores, the five to twelvesecond cores each being surrounded by the clad, wherein the clad has adiameter of 230 μm or less, the first cores, the second cores and theclad are made of glass, and the refractive index of the clad is lowerthan the refractive index of the first cores and the refractive index ofthe second cores, wherein the clad comprises: inner clad layers eachsurrounding corresponding one of the first cores and the second coresand having a lower refractive index than the refractive index of thecorresponding one of the first cores and the second cores; and lowrefractive index layers each surrounding corresponding one of the innerclad layers and having a lower average refractive index than therefractive index of the corresponding one of the inner clad layers andthe refractive index of the clad.
 2. The multicore fiber forcommunication according to claim 1, wherein each of the low refractiveindex layers is made of a material having a lower refractive index thanthe refractive index of the clad and the refractive index of thecorresponding one of the inner clad layers.
 3. The multicore fiber forcommunication according to claim 1, wherein, in each of the lowrefractive index layers, a plurality of low refractive index portionswhich have lower refractive indices than the refractive index of theclad and the refractive index of the corresponding one of the inner cladlayers is formed surrounding the corresponding one of the inner cladlayers.
 4. The multicore fiber for communication according to claim 3,wherein the low refractive index portion is a hole.
 5. The multicorefiber for communication according to claim 1, wherein the clad has adiameter of 165 μm or more.